The invention relates to an improved process for automatic drive slip control in a vehicle with rear wheel drive.
Most of today's vehicles are equipped with an anti-lock brake system (ABS), as well as an automatic drive slip control (often referred to as Anti-Spin Regulation or ASR). The ABS/ASR may be part of a conventional braking system or part of an electronic braking system (EBS).
ABS/ASR systems are used to prevent locking or slipping of the wheels during braking or starting of a vehicle by limiting the slip of the wheels to an optimal desired value. For example, in a vehicle equipped with an ASR system, the permissible slip of the drive wheels is limited to a value of approximately 10%. In order to accomplish this, the wheel speeds of the vehicle are detected by means of rotational speed sensors which produce signals representative of the wheel speeds. The signals are transmitted to an electronic system where the signals are processed by means of suitably programmed microprocessors. The electronic system then transmits output signals to solenoid valves which control the brakes in order to influence the wheels which are beginning to lock or slip.
In a vehicle with a known ASR system where only one of the drive wheels is slipping, only the drive wheel which is slipping is braked. As a result, torque is transmitted via a differential gear on the drive axle to the other wheel on the drive axle. However, if both drive wheels are slipping, the ASR additionally lowers the engine speed or engine power to prevent further slippage. A more detailed description of such a known ABS/ASR system is contained, for example, in the WABCO publication entitled: “Das integrierte Sicherheitssystem für Nutzfahrzeuge, Anti-Blockier-System ABS mit Antriebs-Schlupf-Regelung ASR” (“Anti-lock Brake System ABS With Integrated Drive Slip Control ASR For Commercial Vehicles”), March 1987, §§5-5.3, which is incorporated herein by reference.
It is especially difficult for the ASR system to operate properly during starting and continued travel in difficult terrain, e.g., in deep snow or slush. If both drive wheels slip and the engine power is lowered, this reduced engine power may no longer be sufficient to drive the vehicle. In this case, one could imagine to manually increase the normal ASR slip value of the drive wheels by an operator-actuated push-button (“ASR push-button”).
A further method to adapt the ASR slip value automatically to different road conditions and frictional coefficient values of the road is disclosed, for example, in DE-A1 196 03 677. In this case, the ASR slip value is increased on roads with high coefficient of friction values so that the acceleration of the vehicle can thereby be increased.
In this known method, the ASR slip value is increased when excessive fluctuations in dynamic values, e.g., the wheel speeds, of the wheels on the drive axle occur. To accomplish this, a control system is provided which recognizes high-frequency, low amplitude oscillations in the wheels by evaluating the accelerations of the drive wheels. However, the method described in DE-A1 196 03 677 does not take into account the extreme road conditions noted above. With this known method, the ASR slip value can be varied by approximately between 5% and 20% where the higher slip values are used for more adherent road surfaces.
A disadvantage to this method is that the high-frequency oscillations of the drive wheels may occur independently so that the ASR slip value may be influenced thereby in an undesirable manner. This is especially true for vehicles with inserted axles.
It is an object of the present invention to provide a method for improved traction of a vehicle traveling in deep snow or other difficult types of terrain.